1. Field of the Invention
The present invention relates to an endoscope apparatus which periodically irradiates a subject with illuminating light to produce a video signal from imaging signals at an illuminating period and a non-illuminating period.
2. Description of the Related Art
Recently, there has come into use an electronic endoscope apparatus provided with a solid state imaging device, such as a CCD or MOS at the tip part of the endoscope, or endoscope apparatus fitted with a TV camera at the eyepiece part of the endoscope. The aforesaid endoscope apparatus observes the body cavity by inserting an elongated insertable part into the body cavity or, for example, enabling laser treatments to stop bleeding or to cauterize malignant tissue, such as cancer by inserting a laser device into treatment tool channels.
Cautery treatments using the laser device previously have set laser output and a laser applying time based on the result of animal experiments or theoretical experiments to pursue clinical examination. In the case of a treatment in the body cavity, when a solid state imaging device was exposed to light, the luminance of an observation image of the treatment part rose so as to produce a partially white picture, called "whiteout," because the intensity of a laser beam was too strong. Thus, a real-time cauterizing state could not be observed. Therefore, there was a danger of bleeding or perforating by deep cauterization, or there was also danger of cauterizing the part other than the treatment part. To the contrary, the part to be cauterized was insufficiently cauterized, so that the treatment was incomplete.
Then, in order to prevent the whiteout of an observation image in the aforesaid treatment part, for example, when an ND-YAG laser is used, it has been proposed to provide a laser beam cut filter for eliminating infrared rays in an objective optical system and to remove the effect of a YAG laser beam on the observation image.
However, the intensity of the laser beam was strong, and infrared rays in extensive wavelength ranges were generated when the cautery part was burned during the period in which the laser beam was applied. Therefore, it was impossible to eliminate all of these infrared rays by the laser beam cut filter. Also, when the cautery part begins to burn, not only the image became "whited-out" but also the cauterization did not have a favorable effect on the treatment.
Further, the aforesaid YAG laser beam is invisible because it is an infrared laser beam, so that a visible light (such as a He--Ne laser) laser probe for identifying the treatment part is generally provided for guiding light on the same axis as in a YAG laser probe. Since the intensity of the guiding light is constant, the ratio of illuminating light to guiding light changes with endoscopes to be used. Thus, which part is guided cannot be seen clearly when the intensity of the guiding light is much weaker than the illuminating light, and whiteout occurs when the intensity of the guiding light is much stronger than the illuminating light.
In order to solve the aforesaid problem, for example, the invention in which a filter for attenuating guiding light can be selectively used is indicated in the Gazette of Japanese Patent Laid-Open No. 266049/1987. However, in this reference, whiteout of a solid state imaging device cannot be completely prevented because, for example, a cautery part starts burning. Also, an endoscope apparatus in which a laser beam is not sent out during an imaging period of imaging means by radiation of illuminating light by synchronizing the radiation of the illuminating light and the radiation of a laser beam is proposed in the Gazette of Japanese Patent Laid-Open No. 94644/1986. However, a smearing of a solid state imaging device is generated, so that the device has a problem of reducing the quality of the images.
As mentioned above, in the endoscope apparatuses currently in use, when a laser beam and guiding light were sent out when a part is treated using a laser device, a solid state imaging device was exposed to light, so that whiteout might occur at the treatment. Also, when observation by X-ray irradiation and treatment by ultrasonic waves were used at the same time, the X-rays and ultrasonic waves had been superimposed on an observation image of a CCD or the like as noise. In addition, when the endoscope apparatus was used in a place easily affected by outdoor daylight, the contrast of the image had declined and colors of the image had deteriorated. Thus, there were cases in which a real-time subject image from the endoscope could not be observed clearly.